The invention refers to a method for making a master usable for replication of optical storage media of the CD type and including a flat substrate and a photosensitive coating having tracks produced with depressions, the so-called pits, the length of which as measured in track direction, and the mutual distance of which, the so-called lands, representing binary coded data. This generally known process includes i.a. the following steps:
The photosensitive coating is exposed trackwise to a laser beam which is activated by the H-level of a control signal and cut by its L-level; the control signal is the electrical image of a serial, binary data flow containing the data being stored. PA1 The substrate is developed until the photosensitive coating includes depressions at the exposed spots PA1 The development is terminated as soon as the depressions corresponding to the H-levels of the control signal reach a mean volume which equals a given value.
In this process, the control signal generally designated as MTF signal is received from an encoder and transmitted to a laser beam recorder. The encoder converts the data to be stored, e.g. a PCM-coded audio signal, in accordance to the cross interleave Reed-Salomon code with subsequent eight-to-fourteen modulation and addition of control bits and synchronizing bits into the MTF signal which has a so-called channel-bit frequency of few megahertz, currently 4,3218 MHz and includes only nine discrete values of the pulse duration and pulse pause which leads accordingly to the generation of nine different pit lengths and land lengths upon the master. Since the pits and the lands are equivalent and defined as logic value zero while the transitions from pit to land and from land to pit represent the logic one and since in the context of mastering the generation and configuration of the pits constitutes the crucial step, the subsequent data are based upon the pits. The nine pits of different duration and accordingly different lengths are designated as I3 pits to I11 pits. Calculated from the above stated channel bit frequency as reciprocal value is the pit cycle of 231.39 ns and thus the duration of I3 pits as 3.times.231.39 ns=694 ns as well as the duration of the I11 pit as 11.times.231.39 ns=2545 ns.
In order to keep the channel-bit frequency constant, the speed of the master is respectively readjusted during exposure of the master from inside towards the outside. Taking into consideration the currently common storage media of the CD type with a track readout speed of either 1.2 m/s or 1.4 m/s, the speed thus varies between the smallest and the greatest exposed radius of the master either from 486 to 196 rpm (for 1.2 m/s) or from 568 to 228 rpm (for 1.4 m/s). Considering the above stated time periods, the smallest length of a I3 pit for the smaller track read out speed is calculated as 0.833 .mu.m and the greatest length of a I11 pit for the greater track read out speed is calculated as 3.563 .mu.m.
The complex coding process resulting in the MTF signal provides i.a. that this MTF signal is equivalent-free which means that its time-base mean value (based on alternating current) equals zero. In order to meet the same condition for the MTF signal generated by the respective optical storage medium during readout, the I3 pits to I11 pits must have the same geometric lengths as the corresponding I3 lands to I11 lands upon the optical storage medium produced from the exposed master through several, conventional intermediary steps. To attain this, it is necessary to precisely control the development process and to terminate (abort) when the pits have reached a length at which empirically the above stated symmetry condition is met for the finished storage medium, designated short as symmetry =0. Since the length of the pits cannot be directly measured, instead a certain area of the master is illuminated during developing and in the reflected defraction pattern the intensity of the maximum zero order is compared with the one of the maximum of the first order, which as a result corresponds to a measurement of the mean volume of the pits in the illuminated area of the master. When the measured ratio reaches a preset value, the development is aborted.
The development process and the dimensions of the pits produced thereby depend on numerous parameters, e.g. on the electric and optical tolerances of the laser beam recorder, on the composition and the layer thickness of the photosensitive coating and on the aftertreatment of the master. In case, these parameters require a relatively long time for development until fulfilling the symmetry condition, the produced pits are relatively wide and have steep flanks. In the reversed case, a brief time for development results in narrow pits with flat flanks. This correlation is known, compare DE-C2-29 35 789. Through modification of the numerical aperture of the exposing laser beam, the pit width (and simultaneously to a very small degree also the pit length) may be influenced since a great numerical aperture results in a small pit width, and vice versa. However, there is no possibility to targetly influence the steepness of the flanks of the pits. It follows thereby that steep flanks result during readout of a storage medium replicated from such a master in better detectable and in zero/one transitions which are based on time more exactly positioned, and thus leads to a lower block error rate as well as to a lower data jitter while fiat flanks improve the molding capability and possibly facilitate the track tracing during readout.